Intra prediction exploits spatial correlation within a picture or within a picture region. In order to improve coding efficiency, the High-Efficiency Video Coding (HEVC) standard being developed exploits block-based spatial prediction extensively. In HEVC, multiple Intra prediction modes are used to exploit spatial features and the number of Intra prediction modes depends on the block size of a Prediction Unit (PU). The size of the PU for Intra coding can be 64×64, 32×32, 16×16, 8×8, or 4×4. The number of Intra prediction modes can be as many as 35, including 33 directional prediction modes, one DC mode and one Planar mode as shown in FIG. 1A and FIG. 1B. The directions are labeled with their respective physical directions (H referring to Horizontal direction and V referring to Vertical direction) in FIG. 1A and the directions are labeled with an example of their respective Intra mode indices in FIG. 1B. The correspondences between the physical directions and the Intra mode indices are shown in FIG. 2.
In order to improve compression efficiency, a coding scheme based on most probable mode (MPM) is adopted by HEVC to encode the Intra mode syntax. According to the MPM-based coding scheme, the Intra prediction mode (or Intra mode for abbreviation) for the current block (i.e., the current PU) is predicted using the Intra prediction modes of the left neighboring block and the top neighboring block. In HEVC Test Model Version 5.0 (HM-5.0), two most probable modes are derived from the Intra prediction modes of the left neighboring block and the top neighboring block. If these two Intra prediction modes are the same or one is not available, another Intra mode (Planar or DC mode) is added as the second most probable mode. Because there is a high probability that the Intra prediction mode of the current PU is the same as one of the most probable modes, a shorter codeword is used to encode such cases. Otherwise, a longer codeword is used. In HEVC, the number of Intra prediction modes depends on the PU size as shown in Table 1. The 4×4 and 64×64 PUs only use a subset of the 35 Intra prediction modes for the luma component. For the 4×4 PU, 18 Intra prediction modes are used, which include DC mode, Planar mode, and 16 directional modes as indicated by circles in FIG. 1A and FIG. 1B. It is noted that the directional Intra mode corresponding to the H+6 direction (i.e., mode index 18) is left out of the 18 Intra modes for the 4×4 PU as indicated by a square in FIG. 1A and FIG. 1B. It is desirable to design a set of Intra mode candidates that provides full directional coverage. Furthermore, there are 33 remaining modes besides two MPMs. In order to code these 33 modes, at least six bins (bits) have to be used. Since five bins (bits) can only accommodate 32 modes in a fixed-length representation, using six bins (bits) to code the 33 remaining modes is not efficient. Accordingly, in the current HEVC (HM-5.0), the remaining modes are coded using variable length coding. Nevertheless, it is desirable to use fixed length coding to reduce system complexity if the coding efficiency can be maintained or improved. Further, it is desirable to increase the number of most probable modes so that the number of remaining modes can be reduced.
TABLE 1Block sizeNumber of Luma ModeLuma Modes64 × 6440~3 32 × 32350~3416 × 16350~348 × 8350~344 × 4180~17